/*
 * Copyright 2013 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License,
 * version 2.0 (the "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at:
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 */

package io.netty.util;

import io.netty.util.internal.ObjectUtil;
import io.netty.util.internal.PlatformDependent;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * A pool of {@link Constant}s.
 *
 * @param <T> the type of the constant
 */
public abstract class ConstantPool<T extends Constant<T>> {

  private final ConcurrentMap<String, T> constants = PlatformDependent.newConcurrentHashMap();

  private final AtomicInteger nextId = new AtomicInteger(1);

  /**
   * Shortcut of {@link #valueOf(String) valueOf(firstNameComponent.getName() + "#" +
   * secondNameComponent)}.
   */
  public T valueOf(Class<?> firstNameComponent, String secondNameComponent) {
    if (firstNameComponent == null) {
      throw new NullPointerException("firstNameComponent");
    }
    if (secondNameComponent == null) {
      throw new NullPointerException("secondNameComponent");
    }

    return valueOf(firstNameComponent.getName() + '#' + secondNameComponent);
  }

  /**
   * Returns the {@link Constant} which is assigned to the specified {@code name}. If there's no
   * such {@link Constant}, a new one will be created and returned. Once created, the subsequent
   * calls with the same {@code name} will always return the previously created one (i.e.
   * singleton.)
   *
   * @param name the name of the {@link Constant}
   */
  public T valueOf(String name) {
    checkNotNullAndNotEmpty(name);
    return getOrCreate(name);
  }

  /**
   * Get existing constant by name or creates new one if not exists. Threadsafe
   *
   * @param name the name of the {@link Constant}
   */
  private T getOrCreate(String name) {
    T constant = constants.get(name);
    if (constant == null) {
      final T tempConstant = newConstant(nextId(), name);
      constant = constants.putIfAbsent(name, tempConstant);
      if (constant == null) {
        return tempConstant;
      }
    }

    return constant;
  }

  /**
   * Returns {@code true} if a {@link AttributeKey} exists for the given {@code name}.
   */
  public boolean exists(String name) {
    checkNotNullAndNotEmpty(name);
    return constants.containsKey(name);
  }

  /**
   * Creates a new {@link Constant} for the given {@code name} or fail with an {@link
   * IllegalArgumentException} if a {@link Constant} for the given {@code name} exists.
   */
  public T newInstance(String name) {
    checkNotNullAndNotEmpty(name);
    return createOrThrow(name);
  }

  /**
   * Creates constant by name or throws exception. Threadsafe
   *
   * @param name the name of the {@link Constant}
   */
  private T createOrThrow(String name) {
    T constant = constants.get(name);
    if (constant == null) {
      final T tempConstant = newConstant(nextId(), name);
      constant = constants.putIfAbsent(name, tempConstant);
      if (constant == null) {
        return tempConstant;
      }
    }

    throw new IllegalArgumentException(String.format("'%s' is already in use", name));
  }

  private static String checkNotNullAndNotEmpty(String name) {
    ObjectUtil.checkNotNull(name, "name");

    if (name.isEmpty()) {
      throw new IllegalArgumentException("empty name");
    }

    return name;
  }

  protected abstract T newConstant(int id, String name);

  @Deprecated
  public final int nextId() {
    return nextId.getAndIncrement();
  }
}
